In the peripheral nervous system, all internal organs innervated by the parasympathetic nervous system have muscarinic cholinergic receptors (mAChrs). For example, the heart, gastrointestinal tract, urinary bladder, sweat glands, lacrimal glands, blood vessels, and pupils are all innervated through muscarinic receptors. The central nervous system is also comprised of a complex network of muscarinic receptors, both pre- and postsynaptic. In particular, neurotransmission of acetylcholine is effected by means of muscarinic cholinergic receptors. Muscarinic cholinergic receptors in the brain mediate some of the effects of acetylcholine and cholinergic drugs, and are implicated in many CNS diseases.
Muscarinic cholinergic receptors constitute a family of related proteins. At present, five different mACHr proteins have been cloned and sequenced from the human and rat geonomes; three are known to correspond to pharmacologically defined proteins. See Palacios, J. M. et al., "Cholinergic Receptors in the Rat and Human Brain," Progress in Brain Research 84: 243 (1990). These receptors play a vital role in a number of psychological and behavioral responses, such as the mediation of sleep, avoidance behavior, learning, and memory. Eckelman, W. C. et al., "External Imaging of Cerebral Muscarinic Acetylcholine Receptors," Science 223: 291 (1984); Holman, B. L. et al., "Muscarinic Acetylcholine Receptors in Alzheimer's Disease," JAMA 254: 3063 (1985). In addition, many diseases are manifested through elevated or reduced mAChr levels in the brain, for example, Alzheimer's disease and other illnesses characterized by primary degenerative dementia. See, e.g., Weinberger, D. R. et al., "A Comparison of FDG PET and IQNB SPECT in Normal subjects and Patients with Dementia," J. Neuropsychiatry & Clin. Neurosciences 4: 239 (1992); Weinberger, D. R. et al., "The Distribution of Cerebral Muscarinic Receptors In Vivo in Patients with Dementia," Arch Neurol. 48: 239-248 (1992); Holman, B. L. et al., "Muscarinic Acetylcholine Receptors in Alzheimer's Disease," JAMA 254: 3063 (1985).
Tomographic imaging has emerged as a leading diagnostic tool for diagnosing and researching mAChr activity. In particular, single-photon emission-computed tomography (SPECT) has been used with much success in this area. In SPECT, a radioactive imaging agent is introduced into the subject, where it binds to a receptor in the cells of the subject. Typically, the subject's brain is then scanned with a SPECT scanner, such as the GERASPECT, Digital Scintigraphics, Inc., Waltham, Mass., and the presence of the radioactive imaging agent is detected and observed through the resulting images. See Owens J., et al. "Synthesis of (R,R) .sup.123 IQNB," J. Labelled Compounds and Radiopharmaceuticals 31: 45 (1991); Weinberger, D. R. et al., "Neuropsychopharmacological Imaging with SPECT," Clin. Neuropharm (Suppl. 1): 194A (1992); see generally Neumeyer, J. L. et al., ".sup.123 I-2.beta.-Carbomethoxy-3.beta.-(4-iodophenyl) Tropane," J. Med. Chem. 34:3144 (1991).
Where mAChr is the receptor to be examined, the imaging agent typically employed is *IQNB. This diastereomeric compound is a strong muscarinic cholinergenic receptor antagonist having the following formula: ##STR1## wherein C* and C** are chiral atoms, and *I is a radioactive isotope of iodine, such as .sup.123 I. The four isotopes of *IQNB, namely, (R,R), (R,S), (S,R), and (S,S), are all useful in tomographic imaging studies. By convention, (R,S) refers to the configuration wherein C* has a chirality of R and C** has a chirality of S. See Gibson, R. E. et al., "In Vitro and In Vivo Characteristics of [Iodine-125]-3(R)-Quinuclidinyl-(S)-4 iodobenzilate" J. Nucl. Med. 30: 1079 (1989). Similarly, (S,R) refers to the configuration in which C* has a chirality of S and C** has a chirality of R.
It has been observed that the (R,R) diastereomer of *IQNB has high selectivity when binding to muscarinic cholinergic receptor sites, whereas the (S,S) diastereomer has low binding selectivity. See generally Gibson, R. E. et al., "In Vitro and In Vivo Characteristics of [Iodine-125]-3(R)-Quinuclidinyl-(S)-4-iodobenzilate" J. Nucl. Med. 30: 1079 (1989); Gibson, R. E. et al., "The Characteristics of I-125 4-IQNB and H-3 QNB In Vivo and In Vitro," J. Nucl. Med. 24: 214-222 (1984); Eckelman, W. C. et al., "Use of 3-Quinuclidinyl 4-Iodobenzilate as a Receptor Binding Radiotracer," J. Nucl. Med. 26: 637 (1985). Thus, the (S,S) diastereomer may be used as a reference compound for interpreting the tomographic images. Sawanda, Y. et al., "Kinetic Analysis of 3-Quinuclidinyl-4-[.sup.125 I] Iodobenzilate Transport and Specific Binding to Muscarinic Acetylcholine Receptor in Rat Brain In Vivo," Cereb. Blood Flow Metab. 10: 781 (1990). Other radioactive halogens may be used in place of iodine, for example, .sup.78 Br. When a radioisotope of iodine is used, the isotope preferably is .sup.123 I.
At present, the potential of SPECT imaging of muscarinic receptors as a diagnostic and analytical tool has not been fully attained, primarily due to the high cost and difficulty of synthesizing *IQNB. The radioisotopes of iodine have short half-lives, and thus any reactions involving these isotopes ideally should proceed as quickly as possible to avoid radiodecay of the iodine. For example, .sup.123 I has a half-life of only thirteen hours. Ideally, a synthesis of *IQNB should take only a few minutes to complete, thus allowing for in situ synthesis of *IQNB, and should result in a high radiolabelling yield. Preferably, the synthesis should afford a yield of at least 75%.
Although a number of methods of synthesizing *IQNB are known, no method is known that is capable of preparing *IQNB in such a short time with such high radiolabelling yields. For example, with respect to the conventional Wallach triazine approach, this method requires a reaction time of more than one hour at 80.degree. C., and results in a product with a radiolabelling yield only up to about 20%. Rzeszotarski, W. J. et al., "Synthesis and Evaluation of Radioiodinated Derivatives of 1-Azabicyclo [2.2.2] oct-3-yl-.alpha.-4-iodophyenyl-.alpha.-phenylacetate as Potential Radiopharmaceuticals," J. Med Chem. 27: 156 (1984); Cohen, V. I. et al., "Preparation and Properties of (R)-(-)-1-Azabicyclo [2.2.2]-(R)(+)-.varies.-hydroxy-.alpha.-4[.sup.125 I] iodophenyl-2-phenyl Acetate and R-(-)-1-Azabicyclo [2.2.2] oct. 3-yl-(S)-.alpha.-hydroxy-.alpha.-(4-[.sup.125 I] iodophenyl)-.alpha.-phenyl Acetate as Potential Radiopharmaceuticals, J. Pharmaceutical Sciences 78: 833 (1989). Radioiodination of QNB in trifluoroacetic acid has also been reported; however, this process results in a radiolabelling yield of only up to about 10% after 24 hours reaction time. Lee K. S. et al., "Radioiodination of 3-Quinuclidinyl Benzilate Using No-Carrier-Added Concentration of I-125-NaI," J Nucl. Med. 27: 1045 (1986). Yields of up to about 30% have been reported via a copper-assisted nucleophilic exchange method. Owens J., et al. "Synthesis of (R,R) .sup.123 IQNB," J. Labelled Compounds and Radiopharmaceuticals 31: 45 (1991). Recently, yields of up to 60% of .sup.125 IQNB have been reported via a QNB-boronic acid approach. Kabalka, G. W. et al., "Synthesis of Iodine-125-Labeled-3-Quinuclidinyl 4'-Iodobenzilate," Nucl. Med. Biol. 16: 359 (1989). This method, however, requires a reaction time of one hour, and still fails to attain satisfactory yields.
As a result of the shortcomings of known methods of *IQNB preparation, the cost of *IQNB has made SPECT imaging prohibitive for many laboratories and hospitals. Thus, a clear need exists for a new method of synthesizing *IQNB that overcomes these drawbacks, particularly, the long reaction times and low radiolabelling yields associated with known methods. The present invention seeks to provide such a method, as well as compounds useful in such a method.